I. Glycyl tRNA synthetase. A study of glycyl tRNA synthetase has revealed that 2 forms of this enzyme are interconverted in yeast cells as part of the operation of a regulatory mechanism that should be vital in governing the synthesis of proteins. The cyclic interconversion is modulated by oxygen and promises to be general for all or most aminoacyl tRNA synthetases of many species. II. Evolution. Evidence has been obtained that a number of 5'- substituted uracils can act as "primitive codons" for amino acids having corresponding side chains. This evidence supports a theory on the origin of the genetic code that was proposed earlier.